hydrogen VS. helium

because air in air ship need to be heated,
if we heat hydrogen, it will explose.
As hydrogen react with oxygen,
and heat released.
but helium is a noble gas, so it is very low reactive.
and it will not react with oxygen or any gases in the atmosphere,
so it will be suitable to use in the air ship.

經過 "輕登保" 意外后，人們己停用hydrogen 為飛船入氣。H2 + O2 + heat ==> 爆炸

基本上只要該氣体比空氣輕便可為飛船入氣。

而空氣中約70%是N2、20%是O2，那么要比它們輕的氣体只有几種，而helium 是noble gas最穩定，故用此為飛船入氣。

用helium 之Air ship 並不是加熱使氣体膨脹，而是helium本身比空氣輕。若加熱使氣体膨脹令air-ship升空，基本上空氣已足夠，不用另加其他氣。

this is simple

hydrogen and helium are both lighter than air , so physically both of them can be used as filling gas of air ship. However, hydrogen is dangerous gas due to its explosive property. Thus, hydrogen wont be use for "air ship" purpose

hope can help you

It is because helium is noble gas and will not have chemical reaction with air.

If use hydrogen instead, it will have chemical reaction with air and cause explosion and it will be very dangerous.